Soil precompression stress: II. A comparison of different compaction tests and stress–displacement behaviour of the soil during wheeling

Precompression stress is often used as a criterion for soil susceptibility to compaction. The objective of this study was to compare precompression stresses derived from different compression test methods and relate these values to measured stress and displacement during wheeling in the field. Precompression stress was measured at three depths at two sites on a Eutric Cambisol in Sweden using in situ plate sinkage test, and by compressing soil cores in the laboratory with sequential loading (oedometer) and at constant displacement speed, respectively. At the same sites, wheeling experiments were carried out where vertical soil displacement and vertical soil stress were measured simultaneously at three depths. No clear relationship between precompression stress and compression method was found. Precompression stress values derived from the oedometer and the in situ plate sinkage test did generally not differ from each other. Precompression stress derived from the constant speed test was either higher (site A, silty clay loam) or lower (site B, silty clay) compared to the other methods. Precompression stress did not work as a threshold value between reversible and irreversible deformation when precompression stress values derived from the different methods were compared with stress and displacement measured in the field during the wheeling experiment. Even when the observed stress was lower than the precompression stress, a residual displacement was observed. The study demonstrates that the precompression stress is not a sharp value but depends on the compression test method and its analysis, and that compression tests might not represent the soil behaviour in the field sufficiently. There is an urgent need to study soil behaviour in the field and link these experiments to soil mechanical (laboratory) tests.